Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include portable computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code, system data such as a basic input/output system (BIOS), and other firmware can typically be stored in flash memory devices. Most electronic devices are designed with a single flash memory device.
Block lock bits have typically been used in flash memory devices to gate erase and program operations of the memory array. For example, if the lock bit for a particular memory block has been set, that block cannot be erased or programmed.
A master block lock bit can then be used to gate the block lock bit modifications. In other words, if the master block lock bit is set, a block lock bit that has been set to protect a block of memory cannot be reset by a memory device user. The master block lock bit protects portions of the memory from unauthorized access.
The master block lock bit may be reset by special test mode operations performed at the factory. This provides the factory with the capability of testing the operation of the block lock bits and the master block lock bit. The capability of resetting the master block lock bit is typically not made available to unauthorized users.
However, it may be possible for an unauthorized user to replicate the test mode instructions in order to be able to reset the master block lock bit and thus gain access to the block lock bits. This may be undesirable for a manufacturer that has used a flash memory device to store proprietary data that should not be changed.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved locking scheme for permanently locking master block lock bits.